Unveiling the Viscoelastic Response of Nonequilibrium Low-Entangled Polymer Melt during Its Equilibration

New rheological experiments are presented to investigate the equilibration mechanism of low-entangled, high-molar-mass linear polyethylene upon melting. The evolution of linear viscoelastic spectra is monitored over time. Though the plateau modulus and the conventional crossover point remain stable with time, the terminal flow region exhibits intriguing temporal evolution. A low-frequency second crossover point is observed that moves toward higher moduli and frequencies over time. A phenomenological model is proposed to explain these complex spectra, which correlates strongly with the molecular mechanism proposed by McLeish that rationalizes the kinetics involved in the melting of these materials. We propose a two-step equilibration process. The first step involves equilibration of the nonequilibrium polymer melt, as reflected by the plateau modulus. This equilibration is driven by the mixing of chains upon melting of the low-entangled crystals, resulting in a heterogeneous distribution of topological constraints. This difference in constraints is delineated by a core-shell structure in the polymer melt. The second step addresses the dynamics at larger length scales, suggesting cooperative diffusion of a core-shell heterogeneous structure similar to colloidal dynamics. This second step is more pronounced and experimentally accessible in relatively low molecular weight samples due to the gradual decrease in core size, whereas high molecular weight samples exhibit critical gel-like dynamics at large length scales.